It is known that there are two types of distortion, linear distortion (trapezoidal distortion) and non-linear distortion, that can occur in a projected image projected by a projection apparatus such as a projector. Linear distortion occurs, for example, when the projection apparatus deviates from a position directly opposite to a projection surface. In other words, linear distortion occurs when a projection apparatus designed to be disposed in a position perpendicular to a projection surface is not directly opposite to the projection surface.
Non-linear distortion occurs, for example, when a projection surface such as a hanging projector screen is uneven. In general, an uneven projection surface such as a hanging projector screen lacks linearity, thus non-linear distortion occurs.
A projection system (projector and camera system) that includes a projection apparatus and an image-capturing apparatus is a well-known technology to correct distortion in a projected image. For example, this technology corrects distortion in a projected image projected by the projection apparatus such as a projector on the basis of a captured image captured by a portable image-capturing apparatus (such as a digital camera, a web camera, and a camera built in a mobile phone or a smart phone, and hereinafter referred to as an “external camera”).
Patent document 1 is known as a technical document that discloses a technique to correct distortion in a projected image. Patent document 1 discloses an invention to correct distortion in a projected image by using a captured image captured by a mobile phone with a built-in camera.
However, linear distortion occurs in a corrected projected image in some cases when a captured image acquired by an image-capturing apparatus capturing a projected image is used, for example, in order to correct distortion in the projected image due to unevenness of a projection surface.